This invention relates to a storage cabinet apparatus used in clean rooms in which, for example, semiconductor devices are fabricated, the storage cabinet apparatus being used for temporarily storing works such as wafer cassettes containing semiconductor wafers.
In the fabrication processes of semiconductor devices such as VLSIs and ICs, in particular, in the pretreatment process in which circuit elements are formed on semiconductor wafers, the occurrence of dust in workrooms is a serious obstruction to the desired high yield of semiconductor devices. That is to say, the yield of the semiconductor devices is severely influenced by the cleanliness of the workroom's atmosphere. Therefore, it is preferable that the workrooms used for fabricating semiconductor devices are clean rooms of a cleanliness (which is determined by the number of dust particles contained in a unit volume of atmosphere) higher than a predetermined level. For the same reason, it is required that storage cabinets have the same or higher cleanliness than that of the workrooms. The storage cabinets are cabinets being used in the work rooms for storing wafer cassettes temporarily. Wafer cassettes are stored in these cabinets, for example, when the cassettes are not being used or when the semiconductor wafers contained in the wafer cassettes must wait for the next fabrication process.
FIG. 1 illustrates a typical example of the conventional storage cabinet which has a generally rectangular box-like housing 30 with an elongated entrance 40 extending from side to side. This entrance 40 is provided with a sliding shutter 42 which is driven vertically by a suitable means such as a motor (not shown). In the housing 30, a plurality of horizontal trays 32 are connected by endless chains or sprockets 34 in such a manner that the trays 32 are drawn in an endless line. The trays 32 are adapted to support a plurality of works such as wafer cassettes 44 in which semiconductor wafers are contained. The chains 34 are routed around a plurality of guide wheels 36 rotatably connected to the housing 30. One of the guide wheels 36 is drivingly connected to an electric motor 38 which is secured to the bottom section of the housing 30, and thereby when the motor 38 is actuated, the trays 2 are driven in a direction indicated by the arrows in FIG. 1. In the housing 30, a clean air flow is directed downward to the bottom of the housing 30 or forward to the entrance 40 by a blower so that dust occurring in the housing 30 is immediately discharged out of the housing 30. In order to have the wafer cassettes put into or taken out of the storage cabinet, a desired tray 32 is brought to a position horizontally adjoining the entrance 40. Then, the shutter 42 is opened and the wafer cassettes 44 are put onto or taken off the tray 32 by workers.
However, since the aforementioned conventional storage cabinet is of a structure such that the air flow is directed downward or forward to go through the entire housing 30, dust created in the upper or back area of the inside of the housing 30 is blown throughout the lower or front area of the inside of the housing. That is to say, there arises the potential inconvenience of many semiconductor wafers being contaminated by the dust occurring at one wafer cassette in the housing or at a part of the cabinet such as the chains 34 and the motor 38. Also, since the width of the entrance 40 is as large as the entire length of a tray 32, not only one wafer cassette 44 which is to be put in or taken out of the housing 30 but also all the other wafer cassettes 44 on the same tray 32 are exposed to the external atmosphere upon the opening operation of the shutter 42. This fact may increase the potential of the semiconductor wafers to be contaminated by outside dust such as dust carried by the workers. Although these problems mentioned above can be solved by employing a cassette case which is capable of hermetically encasing each wafer cassette 44, such a cassette case is likely to be an obstacle to the wafer cassettes' automatic transportation which is desired in clean rooms.